Traditionally, tin (II) (stannous) ions are added to oral care compositions to deliver multiple benefits such as anti-microbial effects, control of breath malodor, control of dental plaque growth and metabolism, and reduced gingivitis. However, oral care compositions containing stannous ions (e.g., SnCl2), especially in combination with thickening agents such as sodium carboxymethyl cellulose (CMC), can suffer from poor stability. One of the main reasons for the problem is that Sn2+ ion interacts with anionic polymer CMC. Further, Sn2+ is prone to oxidation causing the oral care composition to exhibit an unacceptably low viscosity. If a formulation routinely decreases in viscosity, such oral care composition can lack phase stability and tends to undergo phase separation over time.
Additionally, oral care compositions, such as a dentifrice, need to balance a number of important health factors such as cleaning, whitening, gum health, and the like with consumer important properties such as pleasant taste and mouth feel. Taste is primarily driven by the type and level of flavor oils included in the formulations. Mouth feel is a result of the rheology and viscosity of the oral care composition.
With taste, for example, consumers like flavors such as peppermint, spearmint, wintergreen, and cinnamon in their oral care compositions. Flavorants are added to the formulation to provide these flavors. These flavorants belong to the class of materials called “volatile oils”, which are generally water-insoluble in aqueous systems at the concentrations needed to provide the desired flavor effects or impact. As a result, solubilizing agents are required. Such solubilizing agents may include: (i) solvents such as ethanol or propylene glycol, and (ii) surfactants such as sodium lauryl sulfate. However, there are challenges with using solvents and surfactants. For example, solvents, particularly at high levels, can impart an unpleasant taste (e.g., bitter, chemical taste) or sensation (e.g., burning). Surfactants used at high levels can impart a bitter or soapy taste and also cause tissue irritation and/or oral cavity desquamation.
With mouth feel, consumers like oral care compositions that are not too runny or too thick and will sit on top of the toothbrush bristles. Polymeric thickeners are commonly used to provide the desirable rheology profile for the oral care compositions. Formulary challenges exist that make it difficult to achieve the desirable mouth feel. For example, too little thickener and the oral care composition can feel watery. Alternatively, too much thickener makes the oral care composition feel gritty in the mouth and difficult to dispense.
A particular challenge for formulating oral care composition that contain essentially water-insoluble components, such as flavor oils, is achieving acceptable phase stability and/or sufficient shelf-life for the product without sacrificing taste and mouth feel. Phase instability tends to be an issue for oral care compositions formulated with flavor oils in combination with other oral care actives via use of emulsions, specifically oil-in-water emulsions. There are two key factors that can impact phase stability of the oral care compositions containing emulsions.
One factor is the viscosity of the external phase (i.e., aqueous phase) surrounding the emulsions. In the case of an oil-in-water emulsion, the decreasing viscosity of the external aqueous phase over time can negatively affect the kinetic stability of the emulsion. For example, the flavor oil molecules can diffuse out of the oil droplet phase into the water phase and then fuse into larger oil droplets. The destabilization caused by the combination of oil droplet collisions and coalescence can lead to the formation of one big oil droplet and the emulsion becoming two separate phases. The time-line for the above reaction can be significantly accelerated in oral care composition having decreased viscosity of the aqueous phase. Current solutions focus on increasing levels of thickeners and/or humectants such as PEGs (see U.S. Patent Publication No. 2013/280182; P&G) to the formulation to modify (i.e., increase) the viscosity of the external aqueous phase and slow down movements of the oil droplets. One disadvantage of using higher levels of thickeners and/or humectants is that it can be expensive. Another disadvantage is that the resulting product can possess an undesirable taste and/or mouth feel.
A second factor that can impact phase stability of the oral care composition is the droplet sizes of the emulsions that form. Droplet sizes impact the emulsions ability to remain kinetically stable over long periods of time. Prior art approaches to control droplet sizes generally focus on controlling processing conditions.
Thus, the need remains for oral care compositions containing flavorants having improved phase stability and/or shelf-life stability over time (i.e., greater than 4 months to 24 months or longer), preferably at ambient conditions. The need also exist for an oral care composition containing flavorants to have physical and chemical stability across a range of manufacturing, handling and storage conditions. It is desirable that the oral care composition, is a dentifrice, and preferably provide pleasant taste and mouth feel experience.